RFR: 8334342: Add MergeStore JMH benchmarks [v6]

[Shaojin Wen]

> [8318446](https://github.com/openjdk/jdk/pull/16245)  brings MergeStore. We need a JMH Benchmark to evaluate the performance of various batch operations and the effect of MergeStore.

Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 14 additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into merge_store_bench
 - Merge remote-tracking branch 'upstream/master' into merge_store_bench
 - bug fix for `putChars4C`
 - bug fix for `putChars4C` and `putChars4S`
 - use VarHandler CHAR_L & CHAR_B
 - copyright
 - bug fix for putIntBU
 - add cases for `getChar` & `putChar`
 - code format
 - add `setIntRL` & `setIntRLU`
 - ... and 4 more: https://git.openjdk.org/jdk/compare/e106b332...74f8837c

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19734/files
  - new: https://git.openjdk.org/jdk/pull/19734/files/4c9b9418..74f8837c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=04-05

  Stats: 48962 lines in 1282 files changed: 31836 ins; 11059 del; 6067 mod
  Patch: https://git.openjdk.org/jdk/pull/19734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19734/head:pull/19734

PR: https://git.openjdk.org/jdk/pull/19734